U.S. patent number 6,621,845 [Application Number 09/981,258] was granted by the patent office on 2003-09-16 for semiconductor laser device which includes algaas optical waveguide layer being formed over internal stripe groove and having controlled refractive index.
This patent grant is currently assigned to Fuji Photo Film Co., Ltd.. Invention is credited to Toshiaki Fukunaga.
United States Patent |
6,621,845 |
Fukunaga |
September 16, 2003 |
Semiconductor laser device which includes AlGaAs optical waveguide
layer being formed over internal stripe groove and having
controlled refractive index
Abstract
In a semiconductor laser device having an InGaAsP compressive
strain quantum well active layer, an InGaAsP first upper optical
waveguide layer formed on the active layer, and a current
confinement layer which is formed above the first upper optical
waveguide layer and includes a stripe groove. An AlGaAs second
upper optical waveguide layer having an approximately identical
refractive index to that of the first upper optical waveguide layer
covers the current confinement layer and the stripe groove. The
product of the strain and the thickness of the active layer does
not exceed 0.25 nm. All the layers other than the compressive
strain quantum well active layer lattice-match with GaAs. An AlGaAs
or InGaAsP upper cladding layer formed above the second upper
optical waveguide layer has an approximately identical refractive
index to that of a lower cladding layer formed under the active
layer.
Inventors: |
Fukunaga; Toshiaki
(Kaisei-machi, JP) |
Assignee: |
Fuji Photo Film Co., Ltd.
(Kanagawa-ken, JP)
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Family
ID: |
18796404 |
Appl.
No.: |
09/981,258 |
Filed: |
October 18, 2001 |
Foreign Application Priority Data
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Oct 18, 2000 [JP] |
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2000-317650 |
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Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
B82Y
20/00 (20130101); H01S 5/2202 (20130101); H01S
5/2209 (20130101); H01S 5/34373 (20130101); H01S
5/3403 (20130101) |
Current International
Class: |
H01S
5/00 (20060101); H01S 5/22 (20060101); H01S
5/343 (20060101); H01S 5/34 (20060101); H01S
005/00 () |
Field of
Search: |
;372/45,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2001-53383 |
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Feb 2001 |
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JP |
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2001053383 |
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Feb 2001 |
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JP |
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Other References
TFujimoto, Yumi Yamada, Yoshikazu Yamada, A. Okubo, Y.Oeda, and K.
Muro "High Power InGaAs/AlGaAs laser diodes with decoupled
confinement heterostructure", 1999, vol. 3628, pp. 38-45..
|
Primary Examiner: Ip; Paul
Assistant Examiner: Nguyen; Tuan
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A semiconductor laser device comprising: a GaAs substrate of a
first conductive type; a lower cladding layer of said first
conductive type having a first refractive index and being formed
above said GaAs substrate; a lower optical waveguide layer formed
above said lower cladding layer; a compressive strain quantum well
active layer made of In.sub.x3 Ga.sub.1-x3 As.sub.1-y3 P.sub.y3 and
formed above said lower optical waveguide layer, where
0<x3.ltoreq.0.4 and 0.ltoreq.y3.ltoreq.0.1; a first upper
optical waveguide layer made of In.sub.x2 Ga.sub.1-x2 As.sub.1-y2
P.sub.y2 and formed above said compressive strain quantum well
active layer, where x2=(0.49.+-.0.01)y2, 0.ltoreq.x2.ltoreq.0.3,
and said first upper optical waveguide layer has a second
refractive index; a first etching stop layer made of In.sub.x9
Ga.sub.1-x9 P of a second conductive type and formed above said
first upper optical waveguide layer, where 0.ltoreq.x9.ltoreq.1; a
second etching stop layer made of In.sub.x1 Ga.sub.1-x1 As.sub.1-y1
P.sub.y1 and formed on said first etching stop layer other than a
stripe area of the first etching stop layer so as to form a first
portion of a stripe groove realizing a current injection window,
where x1=(0.49.+-.0.01)y1 and 0.ltoreq.x1.ltoreq.0.3; a current
confinement layer made of In.sub.0.49 Ga.sub.0.51 P of the first
conductive type and formed above said second etching stop layer so
as to form a second portion of said stripe groove; a second upper
optical waveguide layer made of AlGaAs formed so as to cover said
current confinement layer and said stripe groove; an upper cladding
layer of said second conductive type, made of one of AlGaAs and
In.sub.x4 Ga.sub.1-x4 As.sub.1-y4 P.sub.y4 and formed over said
second upper optical waveguide layer, where x4=(0.49.+-.0.01)y4,
0.9.ltoreq.y4.ltoreq.1; a contact layer of said second conductive
type; a first electrode formed on an exposed surface of said GaAs
substrate; and a second electrode formed on said contact layer;
wherein an absolute value of a first product of a first strain and
a thickness of said compressive strain quantum well active layer is
equal to or smaller than 0.25 nm, and each of said lower cladding
layer, said lower optical waveguide layer, said first and second
upper optical waveguide layers, said first and second etching stop
layers, said current confinement layer, said upper cladding layer,
and said contact layer has such a composition as to lattice-match
with GaAs.
2. A semiconductor laser device according to claim 1, further
comprising first and second tensile strain barrier layers both made
of In.sub.x5 Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 and respectively
formed above and below said compressive strain quantum well active
layer, where 0.ltoreq.x5.ltoreq.0.3 and 0.ltoreq.y5.ltoreq.0.6; and
an absolute value of a sum of said first product and a second
product of a second strain of said first and second tensile strain
barrier layers and a total thickness of the first and second
tensile strain barrier layers is equal to or smaller than 0.25
nm.
3. A semiconductor laser device according to claim 1, wherein said
second etching stop layer is one of said first and second
conductive types.
4. A semiconductor laser device according to claim 1, wherein said
lower optical waveguide is of first conductive type.
5. A semiconductor laser device according to claim 1, wherein said
upper cladding layer consists of one of AlGaAs, InGaAsP and
InGaP.
6. A semiconductor laser device comprising: a GaAs substrate of a
first conductive type; a lower cladding layer of said first
conductive type having a first refractive index and being formed
above said GaAs substrate; a lower optical waveguide layer formed
above said lower cladding layer; a compressive strain quantum well
active layer made of In.sub.x3 Ga.sub.1-x3 As.sub.1-y3 P.sub.y3 and
formed above said lower optical waveguide layer, where
0.ltoreq.x3.ltoreq.0.4 and 0.ltoreq.y3.ltoreq.0.1; a first upper
optical waveguide layer made of In.sub.x2 Ga.sub.1-x2 As.sub.1-y2
P.sub.y2 and formed above said compressive strain quantum well
active layer, where x2=(0.49.+-.0.01)y2, 0.ltoreq.x2.ltoreq.0.3,
and said first upper optical waveguide layer has a second
refractive index; a first etching stop layer made of In.sub.x9
Ga.sub.1-x9 P of a second conductive type and formed above said
first upper optical waveguide layer, where 0.ltoreq.x9.ltoreq.1; a
second etching stop layer made of In.sub.x1 Ga.sub.1-x1 As.sub.1-y1
P.sub.y1 and formed on said first etching stop layer other than a
stripe area of the first etching stop layer so as to form a first
portion of a stripe groove realizing a current injection window,
where x1=(0.49.+-.0.01)y1 and 0.ltoreq.x1.ltoreq.0.3; a current
confinement layer made of In.sub.0.49 Ga.sub.0.51 P of the first
conductive type and formed above said second etching stop layer so
as to form a second portion of said stripe groove; a second upper
optical waveguide layer made of AlGaAs formed so as to cover said
current confinement layer and said stripe groove, wherein said
second upper optical waveguide layer has a third refractive index
which is at most 0.5% different from said second refractive index;
an upper cladding layer of said second conductive type, made of one
of AlGaAs and In.sub.x4 Ga.sub.1-x4 As.sub.1-y4 P.sub.y4 and formed
over said second upper optical waveguide layer, where
x4=(0.49.+-.0.01)y4, 0.9.ltoreq.y4.ltoreq.1 and wherein said upper
cladding layer has a fourth refractive index is at most 0.5%
different from said first refractive index; a contact layer of said
second conductive type; a first electrode formed on an exposed
surface of said GaAs substrate; and a second electrode formed on
said contact layer; wherein an absolute value of a first product of
a first strain and a thickness of said compressive strain quantum
well active layer is equal to or smaller than 0.25 nm, and each of
said lower cladding layer, said lower optical waveguide layer, said
first and second upper optical waveguide layers, said first and
second etching stop layers, said current confinement layer, said
upper cladding layer, and said contact layer has such a composition
as to lattice-match with GaAs.
7. A semiconductor laser device according to claim 6, further
comprising first and second tensile strain barrier layers both made
of In.sub.x5 Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 and respectively
formed above and below said compressive strain quantum well active
layer, where 0.ltoreq.x5.ltoreq.0.3 and 0.ltoreq.y5.ltoreq.0.6; and
an absolute value of a sum of said first product and a second
product of a second strain of said first and second tensile strain
barrier layers and a total thickness of the first and second
tensile strain barrier layers is equal to or smaller than 0.25
nm.
8. A semiconductor laser device according to claim 6, wherein said
second etching stop layer is one of said first and second
conductive types.
9. A semiconductor laser device according to claim 6, wherein said
lower optical waveguide is of first conductive type.
10. A semiconductor laser device according to claim 6, wherein said
upper cladding layer consists of one of AlGaAs, InGaAsP and
InGaP.
11. A semiconductor laser device according to claim 1, wherein a
difference in the equivalent refractive index of the first upper
optical waveguide layer and the second upper optical waveguide
layer between a portion of the active region under the current
injection window and another portion of the active region under the
current confinement layer ranges from 1.5.times.10.sup.-3 to
7.times.10.sup.-3.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device
having a compressive strain quantum well active layer above a GaAs
substrate.
2. Description of the Related Art
Fujimoto et al. ("High Power InGaAs/AlGaAs laser diodes with
decoupled confinement heterostructure," Proceedings of SPIE, Vol.
3628 (1999) pp. 38-45) discloses an internal striped structure
semiconductor laser device which emits light in the 0.98 Mm band.
This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al.sub.x Ga.sub.1-x As lower
cladding layer, an n-type GaAs optical waveguide layer, an InGaAs
quantum well active layer, a p-type GaAs first upper optical
waveguide layer, and an n-type Al.sub.y Ga.sub.1-y As current
confinement layer are formed in this order. Next, a narrow-stripe
groove is formed, by conventional photolithography and selective
etching, to such a depth that the groove penetrates the n-type
AlGaAs current confinement layer. Thereafter, over the above
structure, a GaAs second optical waveguide layer, a p-type AlGaAs
upper cladding layer, and a p-type GaAs contact layer are formed.
Thus, an internal striped structure is formed, and the
semiconductor laser device oscillates in a fundamental transverse
mode.
In the above semiconductor laser device, the stripe width can be
controlled accurately, and high-output-power oscillation in the
fundamental transverse mode can be realized by the difference in
the refractive index between the n-type AlGaAs current confinement
layer and the p-type GaAs second optical waveguide layer. However,
the above semiconductor laser device has a drawback that it is
difficult to form a GaAs layer on another AlGaAs layer, since the
AlGaAs layers are prone to oxidation. In addition, since the
optical waveguide layers are made of GaAs, current leakage is
likely to occur. Therefore, AlGaAs leak-current protection layers
are provided on both sides of the active layer. Nevertheless, the
leakage current is still great, and thus the threshold current is
high.
On the other hand, in order to prevent degradation of
characteristics of the semiconductor laser device due to oxidation
of aluminum included in an exposed regrowth boundary, T. Fukunaga
(the inventor of the present patent application) and M. Wada have
proposed a semiconductor laser device and a method of producing the
semiconductor laser device in a coassigned and copending U.S. Ser.
No. 09/634,703, filed on Aug. 7, 2000 and entitled "HIGH-POWER
SEMICONDUCTOR LASER DEVICE HAVING CURRENT CONFINEMENT STRUCTURE AND
INDEX-GUIDED STRUCTURE," corresponding to Japanese patent
application No. 11(1999)-222169, which is disclosed in Japanese
Unexamined Patent Publication No. 2001-053383. In the above
semiconductor laser device, the optical waveguide layers are made
of InGaAsP, which has a greater bandgap than GaAs and does not
contain aluminum. In addition, the current confinement layer is
made of InGaP. Thus, the semiconductor laser device has a structure
in which aluminum is not exposed on the regrowth layer. However,
even in this structure, the leakage current is still great, and
therefore the threshold current is high, since the band offset
between the conduction bands of the InGaAsP and InGaP layers is
small.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reliable
semiconductor laser device which includes an internal stripe groove
and a regrown layer over an internal stripe groove, and has the
following features: (a) Aluminum, which is prone to oxidation, does
not exist on a regrowth boundary. (b) The leakage current is
suppressed by an index-guided structure formed with high precision.
(c) The semiconductor laser device oscillates in a fundamental
transverse mode when the stripe width is small. (d) The
semiconductor laser device produces low noise when the stripe width
is great.
According to the present invention, there is provided a
semiconductor laser device including: a GaAs substrate of a first
conductive type; a lower cladding layer of the first conductive
type formed above the GaAs substrate; a lower optical waveguide
layer formed above the lower cladding layer; a compressive strain
quantum well active layer made of In.sub.x3 Ga.sub.1-x3 As.sub.1-y3
P.sub.y3 and formed above the lower optical waveguide layer, where
0<x3.ltoreq.0.4 and 0.ltoreq.y3.ltoreq.0.1; a first upper
optical waveguide layer made of In.sub.x2 Ga.sub.1-x2 As.sub.1-y2
P.sub.y2 and formed above the compressive strain quantum well
active layer, where x2=(0.49.+-.0.01)y2, and
0.ltoreq.x2.ltoreq.0.3; a first etching stop layer made of
In.sub.x9 Ga.sub.1-x9 P of a second conductive type and formed
above the first upper optical waveguide layer, where
0.ltoreq.x9.ltoreq.1; a second etching stop layer made of In.sub.x1
Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 and formed on the first etching
stop layer other than a stripe area of the first etching stop layer
so as to form a first portion of a stripe groove realizing a
current injection window, where x1=(0.49.+-.0.01)y1 and
0.ltoreq.x1.ltoreq.0.3; a current confinement layer made of
In.sub.0.49 Ga.sub.0.51 P of the first conductive type and formed
above the second etching stop layer so as to form a second portion
of the stripe groove; a second upper optical waveguide layer made
of A GaAs formed so as to cover the current confinement layer and
the stripe groove; an upper cladding layer of the second conductive
type, made of one of AlGaAs and In.sub.x4 Ga.sub.1-x4 As.sub.1-y4
P.sub.y4 and formed over the second upper optical waveguide layer,
where x4=(0.49.+-.0.01)y4, and 0.9.ltoreq.y4.ltoreq.1; a contact
layer of the second conductive type; a first electrode formed on an
exposed surface of the GaAs substrate; and a second electrode
formed on the contact layer. In the semiconductor laser device, the
first and second upper optical waveguide layers have an
approximately identical refractive index, the upper and lower
cladding layers have an approximately identical refractive index,
the absolute value of a first product of the strain and the
thickness of the compressive strain quantum well active layer is
equal to or smaller than 0.25 nm, and each of the lower cladding
layer, the lower optical waveguide layer, the first and second
upper optical waveguide layers, the first and second etching stop
layers, the current confinement layer, the upper cladding layer,
and the contact layer has such a composition as to lattice-match
with GaAs.
Preferably, the semiconductor laser device according to the present
invention may also have one or a combination of the following
additional features (i) and (ii). (i) The semiconductor laser
device according to the present invention may further include first
and second tensile strain barrier layers both made of In.sub.x5
Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 and respectively formed above and
below the compressive strain quantum well active layer, where
0.ltoreq.x5.ltoreq.0.3 and 0<y5.ltoreq.0.6, and the absolute
value of the sum of the first product and a second product of the
strain of the first and second tensile strain barrier layers and
the total thickness of the first and second tensile strain barrier
layers is equal to or smaller than 0.25 nm. (ii) The second etching
stop layer may be one of the first and second conductive types.
The strain .DELTA.a of the compressive strain quantum well active
layer is defined as .DELTA.a=(ca-cs)/cs, and the strain .DELTA.b of
the first and second tensile strain barrier layers is defined as
.DELTA.b=(cb-cs)/cs, where cs, ca and cb are the lattice constants
of the GaAs substrate, the compressive strain quantum well active
layer, and the first and second tensile strain barrier layers,
respectively.
When a layer grown over the substrate has a lattice constant c, and
the absolute value of the amount .DELTA.=(c-cs)/cs is equal to or
smaller than 0.003, the layer is lattice-matched with the (GaAs)
substrate.
When the thickness of the compressive strain quantum well active
layer is denoted by da, according to the present invention, the
above first product of the compressive strain .DELTA.a and the
thickness da of the compressive strain quantum well active layer
satisfies the following inequalities,
In addition, when the semiconductor laser device according to the
present invention has the additional feature (i), the absolute
value of the sum of the first product and the second product of the
strain .DELTA.b of said first and second tensile strain barrier
layers and the total thickness db of the first and second tensile
strain barrier layers satisfies the following inequalities,
Further, in order to substantially equalize the refractive indexes
of the first and second upper optical waveguide layers, it is
preferable to determine the composition of AlGaAs so that the
difference between the refractive indexes of the first and second
upper optical waveguide layers does not exceed 0.5%.
The semiconductor laser device according to the present invention
has the following advantages.
(a) Because of the above construction, the semiconductor laser
device according to the present invention can oscillate in a
fundamental transverse mode in a wide range from a low output power
to a high output power.
Specifically, in the above semiconductor laser device, a stripe
groove is formed in the In.sub.0.49 Ga.sub.0.51 P current
confinement layer of the first conductive type, and the AlGaAs
second upper optical waveguide layer is formed so as to cover the
current confinement layer and the stripe groove, where the second
upper optical waveguide layer has the refractive index
approximately identical to the refractive index of the first upper
optical waveguide layer. Therefore, it is possible to maintain a
difference in the equivalent refractive index between a portion of
the active region under the current injection window and another
portion of the active region under the current confinement layer in
the range from about 1.5.times.10.sup.-3 to 7.times.10.sup.-3.
Therefore, it is possible to achieve efficient light confinement,
and realize an internal current confinement structure and a real 15
refractive index guided structure with high precision.
(b) Since it is possible to increase the band offset between the
conduction bands of the first and second upper optical waveguide
layers, the leakage current can be suppressed, and oscillation with
low threshold current density can be realized.
(c) When the upper cladding layer is made of AlGaAs having such a
composition that the upper cladding layer has an approximately
identical refractive index to that of the lower cladding layer, the
temperature dependency characteristic of the threshold current can
be improved.
(d) In the semiconductor laser device according to the present
invention, the In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second
etching stop layer is formed under the In.sub.0.49 Ga.sub.0.51 P
current confinement layer, and the second conductive type In.sub.x9
Ga.sub.1-x9 P first etching stop layer is formed under the
In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second etching stop
layer. Therefore, when the current confinement layer is removed by
etching with a hydrochloric acid etchant, the In.sub.x1 Ga.sub.1-x1
As.sub.1-y1 P.sub.y1 second etching stop layer is not removed by
etching with the hydrochloric acid etchant. Thus, the etching with
the hydrochloric acid etchant can be accurately stopped at the
upper surface of the In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1
second etching stop layer.
In addition, when etching with a sulfuric acid etchant is used,
only the second etching stop layer made of In.sub.x1 Ga.sub.1-x1
As.sub.1-y1 P.sub.y1 is etched off, and the In.sub.x9 Ga.sub.1-x9 P
first etching stop layer is not etched. Therefore, the etching with
the sulfuric acid etchant can be accurately stopped at the upper
surface of the In.sub.x9 Ga.sub.1-x9 P first etching stop
layer.
Further, even when a GaAs cap layer is formed on the current
confinement layer, it is possible to concurrently remove the GaAs
cap layer and a portion of the In.sub.x1 Ga.sub.1-x1 As.sub.1-y1
P.sub.y1 second etching stop layer exposed at the bottom of the
stripe groove after the stripe groove is formed.
Furthermore, it is possible to enhance the controllability of the
width of the stripe groove in wet etching, and accurately form the
index-guided structure and the internal current confinement
structure.
(e) Since the current confinement layer is arranged inside the
semiconductor laser device, it is possible to increase the contact
area between the electrode and the contact layer. Therefore, the
contact resistance can be reduced.
(f) Since the layers exposed at the boundary on which the second
etching stop layer is formed do not contain aluminum, regrowth of
the second etching stop layer on the boundary is easy. In addition,
since crystal defects caused by oxidation of aluminum can be
reduced, the degradation of the characteristics of the
semiconductor laser device can be prevented.
(g) When the first and second tensile strain barrier layers both
made of In.sub.x5 Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 are respectively
formed above and below the compressive strain quantum well active
layer, various characteristics of the semiconductor laser device
are improved (e.g., the threshold current is lowered), and
reliability is increased.
DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1D are cross-sectional views of representative stages
of a process for producing a semiconductor laser device as a first
embodiment of the present invention.
FIG. 2 is a cross-sectional view of a semiconductor laser device as
a second embodiment of the present invention.
FIG. 3 is a cross-sectional view of a semiconductor laser device as
a third embodiment of the present invention.
FIG. 4 is a cross-sectional view of a semiconductor laser device as
a fourth embodiment of the present invention.
FIG. 5 is a graph indicating temperature dependencies of threshold
currents in a conventional semiconductor laser device and
semiconductor laser devices as the first and second embodiments of
the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of the present invention are explained in detail below
with reference to drawings.
First Embodiment
FIGS. 1A to 1D are diagrams illustrating cross sections of the
representative stages in the process for producing a semiconductor
laser device as the first embodiment of the present invention.
First, as illustrated in FIG. 1A, an n-type In.sub.0.49 Ga.sub.0.51
P lower cladding layer 12, an n-type or i-type (intrinsic)
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 lower optical waveguide
layer 13 (x2=(0.49.+-.0.01)y2, 0.ltoreq.y2<0.6), an In.sub.x3
Ga.sub.1-x3 As.sub.1-y3 P.sub.y3 compressive strain quantum well
active layer 14 (0<x3.ltoreq.0.4, 0.ltoreq.y3.ltoreq.0.1), a
p-type or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first
upper optical waveguide layer 15, a p-type In.sub.x9 Ga.sub.1-x9 P
first etching stop layer 16 (0.ltoreq.x9.ltoreq.1) having a
thickness of about 10 nm, a p-type In.sub.x1 Ga.sub.1-x1
As.sub.1-y1 P.sub.y1 second etching stop layer 17
(0.ltoreq.x1.ltoreq.0.3, 0.ltoreq.y1.ltoreq.0.3) having a thickness
of about 10 nm, an n-type In.sub.0.49 Ga.sub.0.51 P current
confinement layer 18 having a thickness of about 1 micrometer, and
an n-type GaAs cap layer 19 having a thickness of about 10 nm are
formed on an n-type GaAs substrate 11 by organometallic vapor phase
epitaxy. Then, a SiO.sub.2 film 20 is formed over the n-type GaAs
cap layer 19, and a stripe area of the SiO.sub.2 film 20 having a
width of about 1.5 to 3 micrometers and extending in the
<011> direction is removed by conventional lithography.
Next, as illustrated in FIG. 1B, the n-type GaAs cap layer 19 is
etched with a sulfuric acid etchant by using the remaining areas of
the SiO.sub.2 film 20 as a mask until a stripe area of the n-type
In.sub.0.49 Ga.sub.0.51 P current confinement layer 18 is exposed.
Then, the exposed area of the n-type In.sub.0.49 Ga.sub.0.51 P
current confinement layer 18 is etched with a hydrochloric acid
etchant until a stripe area of the p-type In.sub.x1 Ga.sub.1-x1
As.sub.1-y1 P.sub.y1 second etching stop layer 17 is exposed.
Thereafter, as illustrated in FIG. 1C, the remaining areas of the
SiO.sub.2 film 20 are removed by a fluoric acid etchant. Then, the
remaining areas of the n-type GaAs cap layer 19 and the exposed
area of the p-type In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1
second etching stop layer 17 are removed by etching with a sulfuric
acid etchant so that a stripe area of the p-type In.sub.x9
Ga.sub.1-x9 P first etching stop layer 16 is exposed.
Finally, as illustrated in FIG. 1D, a p-type Al.sub.z2 Ga.sub.1-z2
As second upper optical waveguide layer 21, a p-type In.sub.0.49
Ga.sub.0.51 P upper cladding layer 22, and a p-type GaAs contact
layer 23 are formed over the construction of FIG. 1C. Then, a p
electrode 24 is formed on the p-type GaAs contact layer 23. In
addition, the exposed (opposite) surface of the substrate 11 is
polished, and an n electrode 25 is formed on the polished surface
of the substrate 11. Next, both end surfaces of the layered
construction are cleaved, and a high reflectance coating and a low
reflectance coating are provided on the respective end surfaces so
as to form a resonator. Then, the above construction is formed into
a chip of a semiconductor laser device.
In the above construction, the p-type or i-type In.sub.x2
Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical waveguide
layer 15 and the p-type Al.sub.z2 Ga.sub.1-z2 As second upper
optical waveguide layer 21 have such thicknesses and compositions
that oscillation in a fundamental transverse mode can be maintained
even when output power becomes high. In other words, the p-type or
i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 15 and the p-type Al.sub.z2 Ga.sub.1-z2 As
second upper optical waveguide layer 21 have such thicknesses that
an effective refractive index difference realized by the current
confinement area and the light emission area (i.e., a difference in
the equivalent refractive index between the portion formed in a
stacking direction, i.e., a direction perpendicular to the active
layer, in relation to the current confinement layer and the portion
formed in a stacking direction in relation to the stripe region)
becomes about 1.5.times.10.sup.-3 to 7.times.10.sup.-3.
In addition, in order to realize the symmetry of the oscillation
mode, it is preferable to arrange the p-type Al.sub.z2 Ga.sub.1-z2
As second upper optical waveguide layer 21 to have a refractive
index approximately identical to the refractive index of the p-type
or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 15. Specifically, it is preferable that the
difference in the refractive index between the p-type or i-type
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical
waveguide layer 15 and the p-type Al.sub.z2 Ga.sub.1-z2 As second
upper optical waveguide layer 21 does not exceed 0.5%.
Second Embodiment
FIG. 2 is a cross-sectional view of a semiconductor laser device as
the second embodiment of the present invention.
First, as illustrated in FIG. 2, an n-type In.sub.0.49 Ga.sub.0.51
P lower cladding layer 32, an n-type or i-type (intrinsic)
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 lower optical waveguide
layer 33 (x2=(0.49.+-.0.01)y2, 0.ltoreq.x2.ltoreq.0.3), an
Inx.sub.3 Ga.sub.1-x3 As.sub.1-3 P.sub.y3 compressive strain
quantum well active layer 34 (0<x3<0.4,
0.ltoreq.y3.ltoreq.0.1), a p-type or i-type In.sub.x2 Ga.sub.1-x2
As.sub.1-y2 P.sub.y2 first upper optical waveguide layer 35, a
p-type In.sub.x9 Ga.sub.1-x9 P first etching stop layer 36
(0.ltoreq.x9.ltoreq.1) having a thickness of about 10 nm, a p-type
In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second etching stop
layer 37 (0.ltoreq.x1.ltoreq.0.3, 0.ltoreq.y1.ltoreq.0.3) having a
thickness of about 10 nm, an n-type In.sub.0.49 Ga.sub.0.51 P
current confinement layer 38 having a thickness of about 1
micrometer, and an n-type GaAs cap 10 layer (not shown) having a
thickness of about 10 nm are formed on an n-type GaAs substrate 31
by organometallic vapor phase epitaxy. Then, a SiO.sub.2 film (not
shown) is formed over the n-type GaAs cap layer, and a stripe area
of the SiO.sub.2 film having a width of about 1.5 to 3 micrometers
and extending in the <011> direction is removed by
conventional lithography.
Next, the n-type GaAs cap layer is etched with a sulfuric acid
etchant by using the remaining areas of the SiO.sub.2 film as a
mask until a stripe area of the n-type In.sub.0.49 Ga.sub.0.51 P
current confinement layer 38 is exposed. Then, the exposed area of
the n-type In.sub.0.49 Ga.sub.0.51 P current confinement layer 38
is etched with a hydrochloric acid etchant until a stripe area of
the p-type In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second
etching stop layer 37 is exposed.
Thereafter, the remaining areas of the SiO.sub.2 film are removed
by a fluoric acid etchant. Then, the remaining areas of the n-type
GaAs cap layer and the exposed area of the p-type In.sub.x1
Ga.sub.1-x1 As.sub.y-y1 P.sub.y1 second etching stop layer 37 are
removed by etching with a sulfuric acid etchant so that a stripe
area of the p-type In.sub.x9 Ga.sub.1-x9 P first etching stop layer
36 is exposed.
Finally, a p-type Al.sub.2 Ga.sub.1-z2 As second upper optical
waveguide layer 41, a p-type A10.53Ga0.47As upper cladding layer
42, and a p-type GaAs contact layer 43 are formed over the above
construction. Then, a p electrode 44 is formed on the p-type GaAs
contact layer 43. In addition, the exposed (opposite) surface of
the substrate 31 is polished, and an n electrode 45 is formed on
the polished surface of the substrate 31. Next, both end surfaces
of the layered construction are cleaved, and a high reflectance
coating and a low reflectance coating are provided on the
respective end surfaces so as to form a resonator. Then, the above
construction is formed into a chip of a semiconductor laser
device.
In the above construction, the p-type or i-type In.sub.x2
Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical waveguide
layer 35 and the p-type Al.sub.z2 Ga.sub.1-z2 As second upper
optical waveguide layer 41 have such thicknesses and compositions
that oscillation in a fundamental transverse mode can be maintained
even when output power becomes high. In other words, the p-type or
i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 35 and the p-type Al.sub.z2 Ga.sub.1-z2 As
second upper optical waveguide layer 41 have such thicknesses that
an effective refractive index difference realized by the current
confinement area and the light emission area becomes
1.5.times.10.sup.-3 to 7.times.10.sup.-3.
In addition, in order to realize the symmetry of the oscillation
mode, it is preferable to arrange the p-type Al.sub.z2 Ga.sub.1-z2
As second upper optical waveguide layer 41 to have a refractive
index approximately identical to the refractive index of the p-type
or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 35. Specifically, it is preferable that the
difference in the refractive index between the p-type or i-type
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical
waveguide layer 35 and the p-type Al.sub.z2 Ga.sub.1-z2 As second
upper optical waveguide layer 41 does not exceed 0.5%.
Third Embodiment
FIG. 3 is a cross-sectional view of a semiconductor laser device as
the third embodiment of the present invention.
First, as illustrated in FIG. 3, an n-type Al.sub.z1 Ga.sub.1-z1 As
lower cladding layer 52 (0.35.ltoreq.z1.ltoreq.0.7), an n-type or
i-type (intrinsic) Al.sub.z2 Ga.sub.1-z2 As lower optical waveguide
layer 53 (0.ltoreq.z2.ltoreq.0.2), an In.sub.x5 Ga.sub.1-x5
As.sub.1-y5 P.sub.y5 tensile strain barrier layer 54
(0.ltoreq.x5.ltoreq.0.3, 0.ltoreq.y5.ltoreq.0.6), an In.sub.x3
Ga.sub.1-x3 As.sub.1-y3 P.sub.y3 compressive strain quantum well
active layer 55 (0<x3.ltoreq.0.4, 0.ltoreq.y3.ltoreq.0.1), an
In.sub.x5 Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 tensile strain barrier
layer 56 (0.ltoreq.x5.ltoreq.0.3, 0.ltoreq.y5.ltoreq.0.6), a p-type
or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 57 (x2=(0.49.+-.0.01)y2,
0.ltoreq.x2.ltoreq.0.3), a p-type In.sub.x9 Ga.sub.1-x9 P first
etching stop layer 58 (0.ltoreq.x9.ltoreq.1) having a thickness of
about 10 nm, a p-type In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1
second etching stop layer 59 (x1=(0.49.+-.0.01)y1,
0.ltoreq.x1.ltoreq.0.3) having a thickness of about 10 nm, an
n-type In.sub.0.49 Ga.sub.0.51 P current confinement layer 60
having a thickness of about 1 micrometer, and an n-type GaAs cap
layer (not shown) having a thickness of about 10 nm are formed on
an n-type GaAs substrate 51 by organometallic vapor phase epitaxy.
Then, a SiO.sub.2 film (not shown) is formed over the n-type GaAs
cap layer, and a stripe area of the SiO.sub.2 film having a width
of about 1.5 to 3 micrometers and extending in the <011>
direction is removed by conventional lithography.
Next, the n-type GaAs cap layer is etched with a sulfuric acid
etchant by using the remaining areas of the SiO.sub.2 film as a
mask until a stripe area of the n-type In.sub.0.49 Ga.sub.0.51 P
current confinement layer 60 is exposed. Then, the exposed area of
the n-type In.sub.0.49 Ga.sub.0.51 P current confinement layer 60
is etched with a hydrochloric acid etchant until a stripe area of
the p-type In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second
etching stop layer 59 is exposed.
Thereafter, the remaining areas of the SiO.sub.2 film are removed
by a fluoric acid etchant. Then, the remaining areas of the n-type
GaAs cap layer and the exposed area of the p-type In.sub.x1
Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second etching stop layer 59 are
removed by etching with a sulfuric acid etchant so that stripe area
of the p-type In.sub.x9 Ga.sub.1-x9 P first etching stop layer 58
is exposed.
Finally, a p-type Al.sub.2 Ga.sub.1-z2 As second upper optical
waveguide layer 63, a p-type In.sub.x4 Ga.sub.1-x4 As.sub.1-y4
P.sub.y4 upper cladding layer 64 (x4=(0.49.+-.0.01)y4,
0.9.ltoreq.y4.ltoreq.1), and a p-type GaAs contact layer 65 are
formed over the above construction. Then, a p electrode 66 is
formed on the p-type GaAs contact layer 65. In addition, the
exposed (opposite) surface of the substrate 51 is polished, and an
n electrode 67 is formed on the polished surface of the substrate
51. Next, both end surfaces of the layered construction are
cleaved, and a high reflectance coating and a low reflectance
coating are provided on the respective end surfaces so as to form a
resonator. Then, the above construction is formed into a chip of a
semiconductor laser device.
In the above construction, the p-type or i-type In.sub.x2
Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical waveguide
layer 57 and the p-type Al.sub.2 Ga.sub.1-z2 As second upper
optical waveguide layer 63 have such thicknesses and compositions
that oscillation in a fundamental transverse mode can be maintained
even when output power becomes high. In other words, the p-type or
i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 57 and the p-type Al.sub.z2 Ga.sub.1-z2 As
second upper optical waveguide layer 63 have such thicknesses that
an effective refractive index difference realized by the current
confinement area and the light emission area becomes
1.5.times.10.sup.-3 to 7.times.10.sup.-3.
In addition, in order to realize the symmetry of the oscillation
mode, it is preferable to arrange the p-type Al.sub.z2 Ga.sub.1-z2
As second upper optical waveguide layer 63 to have a refractive
index approximately identical to the refractive index of the p-type
or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 57. Specifically, it is preferable that the
difference in the refractive index between the p-type or i-type
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical
waveguide layer 57 and the p-type Al.sub.2 Ga.sub.1-z2 As second
upper optical waveguide layer 63 does not exceed 0.5%.
Although the GaAs cap layer is formed in the first to third
embodiments, the semiconductor laser device according to the
present invention can be produced without a cap layer. When a GaAs
cap layer is formed as in the first to third embodiments, it is
possible to prevent formation of a natural oxidation film on the
InGaP current confinement layer, and metamorphic change in the
InGaP current confinement layer, which may occur when a resist
layer is formed directly on the InGaP current confinement layer. In
addition, since the GaAs cap layer is removed before the second
upper optical waveguide layer is formed, it is possible to remove a
residue left on the regrowth layer on which the second upper
optical waveguide layer is formed, and prevent occurrence of
crystal defects.
Fourth Embodiment
FIG. 4 is a cross-sectional view of a semiconductor laser device as
the fourth embodiment of the present invention.
First, as illustrated in FIG. 4, an n-type In.sub.0.49 Ga.sub.0.51
P lower cladding layer 72, an n-type or i-type (intrinsic)
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 lower optical waveguide
layer 73 (x2=(0.49.+-.0.01)y2, 0.ltoreq.x2 .ltoreq.0.3), an
In.sub.x5 Ga.sub.1-x5 As.sub.1-y5 P.sub.y5 tensile strain barrier
layer 74 (0.ltoreq.x5.ltoreq.0.3, 0.ltoreq.y5.ltoreq.0.6), an
In.sub.x3 Ga.sub.1-x3 As.sub.1-y3 P.sub.y3 compressive strain
quantum well active layer 75 (0.ltoreq.x3.ltoreq.0.4,
0.ltoreq.y3.ltoreq.0.1), an In.sub.x5 Ga.sub.1-x5 As.sub.1-y5
P.sub.y5 tensile strain barrier layer 76 (0.ltoreq.x5.ltoreq.0.3,
0.ltoreq.y5.ltoreq.0.6), a p-type or i-type In.sub.x2 Ga.sub.1-x2
As.sub.1-y2 P.sub.y2 first upper optical waveguide layer 77
(x2=(0.49.+-.0.01)y2, 0.ltoreq.x2.ltoreq.0.3), a p-type In.sub.x9
Ga.sub.1-x9 P first etching stop layer 78 (0.ltoreq.x9.ltoreq.1)
having a thickness of about 10 nm, a p-type In.sub.x1 Ga.sub.1-x1
As.sub.1-y1 P.sub.y1 second etching stop layer 79
(0.ltoreq.x1.ltoreq.0.3, 0.ltoreq.y1.ltoreq.0.3) having a thickness
of about 10 nm, and an n-type In.sub.0.49 Ga.sub.0.51 P current
confinement layer 80 having a thickness of about 1 micrometer are
formed on an n-type GaAs substrate 71 by organometallic vapor phase
epitaxy. Then, a SiO.sub.2 film (not shown) is formed over the
n-type In.sub.0.49 Ga.sub.0.51 P current confinement layer 80, and
a stripe area of the SiO.sub.2 film having a width of about 1.5 to
3 micrometers and extending in the <011> direction is removed
by conventional lithography.
Next, the n-type In.sub.0.49 Ga.sub.0.51 P current confinement
layer 80 is etched with a hydrochloric acid etchant by using the
remaining areas of the SiO.sub.2 film as a mask until a stripe area
of the p-type In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second
etching stop layer 79 is exposed.
Thereafter, the remaining areas of the SiO.sub.2 film are removed
by a fluoric acid etchant. Then, the exposed area of the p-type
In.sub.x1 Ga.sub.1-x1 As.sub.1-y1 P.sub.y1 second etching stop
layer 79 is removed by etching with a sulfuric acid etchant so that
a stripe area of the p-type In.sub.x9 Ga.sub.1-x9 P first etching
stop layer 78 is exposed.
Finally, a p-type Al.sub.z2 Ga.sub.1-z2 As second upper optical
waveguide layer 82, a p-type In.sub.x4 Ga.sub.1-x4 As.sub.1-y4
P.sub.y4 upper cladding layer 83 (x4=(0.49.+-.0.01)y4,
0.9.ltoreq.y4.ltoreq.1), and a p-type GaAs contact layer 84 are
formed over the above construction. Then, a p electrode 85 is
formed on the p-type GaAs contact layer 84. In addition, the
exposed (opposite) surface of the substrate 71 is polished, and an
n electrode 86 is formed on the polished surface of the substrate
71. Next, both end surfaces of the layered construction are
cleaved, and a high reflectance coating and a low reflectance
coating are provided on the respective end surfaces so as to form a
resonator. Then, the above construction is formed into a chip of a
semiconductor laser device.
In the above construction, the p-type or i-type In.sub.x2
Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical waveguide
layer 77 and the p-type Al.sub.z2 Ga.sub.1-z2 As second upper
optical waveguide layer 82 have such thicknesses and compositions
that oscillation in a fundamental transverse mode can be maintained
even when output power becomes high. In other words, the p-type or
i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 77 and the p-type Al.sub.z2 Ga.sub.1-z2 As
second upper optical waveguide layer 82 have such thicknesses that
an effective refractive index difference realized by the current
confinement area and the light emission area becomes
1.5.times.10.sup.-3 to 7.times.10.sup.-3.
In addition, in order to realize the symmetry of the oscillation
mode, it is preferable to arrange the p-type Al.sub.z2 Ga.sub.1-z2
As second upper optical waveguide layer 82 to have a refractive
index approximately identical to the refractive index of the p-type
or i-type In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper
optical waveguide layer 77. Specifically, it is preferable that the
difference in the refractive index between the p-type or i-type
In.sub.x2 Ga.sub.1-x2 As.sub.1-y2 P.sub.y2 first upper optical
waveguide layer 77 and the p-type Al.sub.z2 Ga.sub.1-z2 As second
upper optical waveguide layer 82 does not exceed 0.5%.
Temperature Dependency of Threshold Current
Temperature dependencies of threshold currents in semiconductor
laser devices according to the present invention are compared with
a temperature dependency of a threshold current in a conventional
semiconductor laser device. Specifically, the semiconductor laser
devices as the first and second embodiments of the present
invention are used in this comparison. In each of the semiconductor
laser devices as the first and second embodiments of the present
invention used in this comparison, the second upper optical
waveguide layers are made of Al.sub.0.11 Ga.sub.0.89 As, the first
upper optical waveguide layers are made of In.sub.0.11 Ga.sub.0.89
As.sub.0.77 P.sub.0.23, and the total thickness of the optical
waveguide layers is 0.8 micrometers. On the other hand, in the
conventional semiconductor laser device used in the comparison, the
second upper optical waveguide layer is made of InGaAsP. All of
semiconductor laser devices as the first and second embodiments of
the present invention and the conventional semiconductor laser
device used in the comparison have a stripe width of 2.5
micrometers and an oscillation wavelength of 1060 nm.
FIG. 5 is a graph indicating the temperature dependencies of the
threshold currents in the conventional semiconductor laser device
and the semiconductor laser devices as the first and second
embodiments of the present invention. As indicated in FIG. 5, the
semiconductor laser devices as the first and second embodiments of
the present invention have lower threshold currents and smaller
temperature dependencies of the threshold current than the
conventional semiconductor laser device.
In addition, the temperature dependency of the threshold current in
the semiconductor laser device as the second embodiment of the
present invention is slightly smaller than that in the
semiconductor laser device as the first embodiment of the present
invention, where the upper cladding layer of the semiconductor
laser device as the second embodiment of the present invention is
made of AlGaAs, and the upper cladding layer of the semiconductor
laser device as the first embodiment of the present invention is
made of InGaP.
Variations and Other Matters
(i) It is possible to form an In.sub.0.49 Ga.sub.0.51 P layer
having a thickness of about 20 nm before the second upper optical
waveguide layer is formed in the process for producing the
semiconductor laser device according to the present invention. In
this case, the leakage current can be more effectively
suppressed.
(ii) Since the temperature dependencies are reduced in the
semiconductor laser devices according to the present invention, and
the semiconductor laser devices can emit a highly reliable laser
beam, the semiconductor laser devices according to the present
invention can be used in the fields of high-speed, information
processing, image processing, communications, laser measurement,
medicine, printing, and the like.
(iii) Since the compressive strain quantum well active layers are
made of In.sub.x3 Ga.sub.1-x3 As.sub.1-y3.sub.P.sub.y3
(0<x3.ltoreq.0.4, 0.ltoreq.y3.ltoreq.0.1), the oscillation
wavelengths of the semiconductor laser devices as the first to
fourth embodiments can be controlled in the range of 900 to 1,200
nm.
(iv) Although n-type GaAs substrates are used in the constructions
of the first to fourth embodiments, instead, p-type GaAs substrates
may be used. When the GaAs substrates are p-type, the conductivity
types of all of the other layers in the constructions of the first
to fourth embodiments should be inverted.
(v) Each layer in the constructions of the first to fourth
embodiments may be formed by molecular beam epitaxy using solid or
gas raw material.
(vi) Although the constructions of the first to fourth embodiments
are index-guided structure semiconductor laser devices, the present
invention can also be used in semiconductor laser devices having a
diffraction grating and optical integrated circuits.
(vii) Although each of the semiconductor laser devices as the first
to fourth embodiments has a stripe width of 1.5 to 3 micrometers,
and oscillates in a fundamental transverse mode, the present
invention can also be applied to broad-stripe index-guided
semiconductor laser devices each having a stripe width of 3
micrometers or more and oscillating in multiple modes. According to
the present invention, it is possible to realize a semiconductor
laser device which produces low noise even in a multimode
operation.
* * * * *